Foam coated paper substrate and process of making same



Oct. 4, 1966 D. FUNCK 3,276,900

FOAM COATED PAPER SUBSTRATE AND PROCESS OF MAKING SAME Filed July 23,1962 FIG. 1

FIG-3 INVENTOR DENNIS LIGHT FUNCK BY W C.

ATTORNEY United States Patent 3,276,900 FOAM COATED PAPER SUBSTRATE ANDPROCESS OF MAKING SAME Dennis Light Funck, Wilmington, Del., assignor toE. L du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware Filed July 23, 1962, Ser. No. 211,697 4Claims. (Cl. 117--93.1)

This invention relates to the production of thin thermoplastic foams ondielectric substrates.

In the past it has been well known that foams of many thermoplasticresins could be produced. However, it has heretofore been impossible toproduce thin foamed sections.

It is an object of this invention to produce such a thin foamed sectionon the surface of a dielectric substrate.

This and other objects are accomplished by subjecting a dielectricsubstrate coated with a thermoplastic resin to rapid dielectric heating.

Thermoplastic films, such as polyethylene films, do not heat when passedthrough a dielectric heater, because they absorb an insuflicient amountof the electric current to effect appreciable heating. The substratesuitable for use in this. invention absorb sufficient electric currentfrom the dielectric to heat rapidly. In the case of cellulose as asubstrate, uncoated cellulose will degrade or burn within a few secondsin a typical dielectric oven or heater. However, a cellulose substratecoated with a thermoplastic resin such as polyethylene will not degradeor burn in such a dielectric oven or heater in less than severalminutes. It appears that the thermoplastic resin coating while notabsorbing the dielectric current absorbs the heat from the substrate andis thereby heated and at the same time reduces the heating effect on thesubstrate in as much as the final temperature reached is reduced.

The requisite properties of the materials used in this invention are asfollows: The coating is a thermoplastic having a softening point belowthe temperature at which the substrate degrades, and yet is not soft atroom temperature. I This requirement as a practical matter means thatthe softening point of the thermoplastic coating is above 40 C. andbelow about 180 C. when using a cellulose substrate.

There also is present in the thermoplastic resin coating H compositionsufiicient volatiles or volatile forming compounds to produce greaterthan 2X10 moles of gas per cc. of thermoplastic resin.. The volatile orvolatileforming compound may be either a low molecular weighthydrocarbon or other volatile. compounds such as water or a chemicalblowing agent which breaks down on heating to form a voltaile componentsuch as carbon dioxide, nitrogen, or water. Absorbed moisture from theatmosphere or remaining monomer naturally occurring in the thermoplasticcoating may be sufiicient so that no special volatile compound orfoaming agent need be added to the interior.

3,276,900 Patented Oct. 4, 1966 The thermoplastic coatings of thisinvention are from 0.5 to 5 mils thick prior to foaming and about 1.5 to30 mils thick after foaming. The increase in thickness of the foamedthermoplastic is from 1.5 to 10 times the thickness of the unfoamedthermoplastic coating. This increase is known as the expansion factor.The final foamed coatings are generally about two bubbles thick. Thethermoplastic surfacings used in this invention may be applied to thesubstrate by any convenient conventional means such as extrusioncoating, lamination, spray coating, dispersion coating or dip coating.

The amount of heat absorbed by a sample in a dielectric oven is governedby the following formula 'wherein the power is expressed in watts.

wherein E is the voltage impressed across the sample, f is the frequencyin cycles per second, K is the dielectric constant of the sample, D isthe dissipation factor of the sample, A is the area of the sampleexposed to the dielectric current in square inches and t is thethickness of the sample in inches. The dielectric substrates used inthis invention have dielectric constants K, of greater than 2.2 anddissipation factors of greater than 0.001. Definitions of K and D arefound in ASTM D-150. ,K=C/C,, where the units for C and C are far'ad/m.The thermoplastic coatings used in this invention have dielectricconstants K and dissipation factors below those of the substrate so asto enable the coating toact as a heat sink. Further informationconcerning dielectric heating may be found in Induction and DielectricHeating by J. W. Cable, Reinhold Publishing Corp., New York, 1954.

The heating effect (power absorbed) is due to the polar groups withinthe molecules attempting to orientate themselves with the electricalfield. Since the field is reversed millions of times a second, such as140 megacycles per second, a considerable amount of electrical energy isconverted into heat by the motion of the molecules. In general, thedielectric field is of at least 25,000 volts and be above 100 megacyclesper second frequency. As can be seen from the equation the more polarthe material the greater is the power absorbed, since the higher theloss factor the more polar is the material. However, it should bepointed out that the larger the air gap between the electrodes, thehigher the voltage needed to impress the same potential across thesample.

In dielectric heating the interior of the material becomes hotter thanthe surfaces. This is in contrast to other types of heating where thesurfaces are hotter than In dielectric heating all portions of a givenmaterial being heated absorb energy at the same rate.

- Since heat is lost from the surfaces to the atmosphere or theelectrodes the surfaces are cooler than the interior. In this way, muchof the foaming is accomplished in the interior of. the structure andvery little of the volatiles creating the foam evaporate compared withconventional heating of such coated substrates. Furthermore, the foamsof this invention have a substantially continuous crust or outer film ofthermoplastic resin.

In the drawings:

FIG. 1 shows a cross-section of a resin coated paper;

FIG. 2 shows a cross-section of the same resin coated paper as FIG. 1after the resin has been foamed in accordance with this invention;

FIG. 3 shows a pair of electrodes arranged to foam a resin coatedsubstrate;

FIG. 4 shows an apparatus for continuously foaming resin coated cups;

- 4 for use in this type of oven, since paper containers would burn,char or ignite before the food was cooked. 'FIG. illustrates the cookingof food 13, in this case a 20 gram sample of frozen chicken in a IO-milthick paper tray FIG. 5 shows a dielectric oven using a resin coatedcon- 5 14 coated with 3 mil polyethylene, in a dielectric oven 15 tainerfor the food being cooked in the oven. ving gr n l r d 1 ,21n p d 1 inchs a ove In FIG. 1 a substrate 1 is shown with a coating 2. The a highvoltage electrode 17, and a front opening WhlCh substrate as abovediscussed is preferably paper and the can if desired, be supphed with adoor not shown. For coating is preferably a polyolefin such aspolyethylene, example, the oven was operated at 2.5 kilovolts at 100polypropylene or blends thereof. In FIG. 2 the same megacycles for 45seconds. The tray as above described substrate 1 is shown but thecoating 2 has been foamed was uninjured and transferred no taste to thechicken by dielectricheating. As can be seen from FIG. 2 the breastscontained therein. The food is heated much coating is preferably about 1to 2 bubbles thick. more rapidly than the polyethylene coated papercontam- FIG. 3 shows ground electrode 3, which for example er becausethe food has a higher dielectric constant K measures 6 inches square and/2 inch thick, supporting than either paper or conventionalthermoplastic resins. the coated substrate 4 prior to heating with theresin The dielectric constants of most foods and especially coating 2facing upwards. High voltage electrode 5 is frozen foods are slightlybelow 78; the dielectric constant located above the coated substrate 4leaving an air gap. of Water compared with the dielectric constant ofpoly- The electrode may b of almost i ethylene is 2.25. The higherdielectric constants of the In FIG 4 a ground electrode 6 is Shownmounted on food indicate that the food will absorb several times the aconveyor belt 7 which in turn contacts roll 8 which heat that thecohtalhel' W111 absorb P unit thickness supplies the belt withelectrical contact. High voltage and therefore h mom than thecontalnerelectrode 9 is also mounted on a belt 10 in contact withthermore, the resm 1f somewhat overheated, will foam roll 11 whichsupplies the high voltage to electrode 9. a thereby expend a Afterfoaming the resin will The speed of the belts and length of their closeparallel Insulate the P p P 9 0f the cohtalhel' from the hot rundetermine the period of residence of thermoplastic food f P chamhg ofthe P p Q yi Iesin coated paper cup 12 within. the dielectric fi 1delectr1c ovens cook food in from 2 to 6 minutes so that The presentinvention involves the realization that a the resin coated Substrates ofthls Invention can with thermoplastic resin, such as polyethylene, whencoated on Stand the heat generated- T ese food cooking ovens norasubstrate, such as paper acts as a heat sink when the v many Operate atfrom watts 5 and from 1,000 structure is placed in a dielectric oven,and absorbs the 5,000 megacycles as Is Set forth on P 484 of Cable, heatgenerated in the paper by the dielectric oven, thereby referred toaboveenabling the structure to withstand several times the Ehamplfis Setforth 111 Table I are Illustrative of amount of dielectric current or anequivalent amount of i j 111 Table I, E/MAA-H represents a freedielectric current for a much longer period of time than catalyzed p yof ethylene and melhacrylic could an uncoated tray or container made ofa mateacld cohlamlflg W1. Percent of methacrylic acid hi h rial Such aspapen copolymer has been treated with 6 wt. percent of hexa- Dielectricovens are commonly used in certain types methylene diamine; E/MAASrepresents a free radical of restaurants or cafeterias, particularlythose which catalyzed p of ethylene and melhacl'yhc acid have automaticcoin operated ovens in which frozen food 40 containing 10 Perceht ofmethacryllc acid which in a tray or container is fed into the oven uponinsertion P y 1138136611 treated with 3 Phmeht of Sodium of a coin andthe oven is automatically turned on. In hydroxide; E/VAC represents acoplymer of ethylene and the past it has been necessary to use heatresistant plasvinyl acetate containing 9 wt. percent of vinyl acetate;tics, metals, or ceramics to fabricate the food container and PMMrepresents polymethyl methacrylate.

TABLE I Coating Coating Additives Ex. Substrate Melt Weight Density,Index, Percent Polymer g./ce. g./10 Based on Additives min. Polymer l 1Kraft Paper Polyethy1ene 0.915 1.9 2 do 0.915 1.9 a Y 0. 915 1.9 40.915 1. 9 2 p, rpoirlybis gbergzeiiiei spl- OIIY Sell]. 08! BZ e 0.9151.9 0' 9 L 9 g AzodDigarbonamide. 0.915 1.9 5 Do. 0.915. 1.9 1003504-21110. 0.915 1.9 7 i0 Na2B407-10H10. 7 0.915 1.9 10 MgCO 0. 923 3.7 2 p, poxybis (benzene sulionyl semlcarbazide) 0.923 3.7 2 casoi-znio.0.923 3.7 2 N8JB407-10H10.

, 2 Azodicarbonamide.

2 D0. 2 Do. 2 Do. 2 Do. 2 D0; 1 2 Do. 22 2 Do. 29 ..(10 2.5 Do. 24..-Polyhexamethylene adipamide.

Expansion Ratio 7 04160770640178329547678 Z3ZZLANZLZLTQWZLRWA ZLAMTZLLBg an article having a continuous last-ic resin comprising the stepCoating, mils Sample Thickness After Treatment Total,

mils

Time, sec.

Estimated Power Voltage Stat, Kv.,

Setting compoun 2 10- mole of gas per cc. of thermoplastic resin andOperating Conditions Electrode Spacing, inches hown in Table II. 30 reTABLE I-Continued mils 66.066053055309612715581IIOLLLZLLLQLLLQKWZZLQLLLE 1 TABLE II Thickness Substrate, Coating, milsSample Dimensions Diameter, inches 1 Each side. The coated substratefoamed in accordance with this invention have high tensile strengths ass This invention is especially useful in foaming thermoplastic resincoatings on paper cups as 1 a m w 6 r. nw m o 0 m m m f m m 1 m l mm t8. C nO h 0 n 1. t 0 S0-$ n wn m mh S m. a WM X h 1 66 W u .monn 3 a moto m S t .1 a%% a e.m d 6 1S In m f m ml 0 a a pd e S a C S Ym 0 ea r ai p a t fl m n h 0 mwa et- .1 r. 4 mOur j wzdt 5 4 d wm met h w .1 m mam m d cups are well suited ing hot liquids such as coffee because 0foam. Furthermore, the foam coate gether as much as unfoamed coated hecups are stacked together.

m m. 0 ck C m i a m f %n e UO h I T P m w 4 PC resin is polyethylene.

References Cited by the Examiner y UNITED STATES PATENTS cups do when tExample 25 A tray formed of IO-mil thick paper coated with a 3-mil thickfilm of polyethylene having a density of .9

5 1 1 X X XX 4 50510 0 u um 6 2 9 9 711771 -12112 1 .1 1.1 2. n u i. n n"m G u "m t aflw wm hGdmkd c k o h o nrselic ea aou h WVBBRWP 55555559999999 1111111 1221777 1 57 76 387405 w fifi flfi m 565914 987749 ,5 31 2 222222 5 5 5 a SO as C m a w mkmw G h hl uu .wF emW h ms m W.1 a .1e mw m r m d.1 a .1 n e pt G Wfld ka e .mn m n mm m uycenfl n lc n 0afim 6 Z a b a m m IVO 1 t r .m a n 51mm 4 P 3 6 t p O a.w k kmmmwi .M w0 w m n S Mmwww whm wmm uP S f. aeo a S D 2 a m mn w wtotianml 3/1960Hirsh 260- 4/1961 Liebeskind.

damage to the tray. 7 I claim:

X 5 XX w 3 5 5 1 nu" a n n n u .m m x n n E t t W ee 0 TSP... m bk P doM EYMC I 333 T 666 R 999 9 A 1111 M 167 D 251 M 713 5 ,3 m naw n E 5 6awammm u flias 5 0 03 enmfl TIO. P 6.6006 Ph n adk 6m k fla wa m .mmwmwm .mmm w mr mompf m ed do ne C H t c a C a .1 a m .Jflef uklanm t0 16m fl. a 0 r f omme m rwm am m m lm P oca u 4 f0 0 "me. O u 0 O n e U u i"5.1 1|. 5 .1 k2 Wmn n n as mu h m nmoimw A 02. e e m Md n m 1 S S.m Ch

resin, and having an expansion factor of from 1.5 to 10, I p 1 whichthermoplastic resin has a softening point between RICHARD NEVIUSExammer' 40 C. and 180 C. C. A. HAASE, T, G. DAVIS, Assistant Examiners.

1. AN ARTICLE OF MANUFACTURE COMPRISING A PAPER SUBSTRATE FROM 2 TO 40MILS THICK AND INTIMATELY ADHERED TO SAID PAPER A SURFACING FROM 1.5 TO30 MILS THICK CONSISTING ESSENTIALLY OF A THERMOPLASTIC RESIN SELECTEDFROM THE CLASS CONSISTING OF POLYETHYLENE AND POLYPROPYLENE FOAM HAVINGA CONTINUOUS OUTER SURFACE OF SAID THERMOPLASTIC RESIN, AND HAVING ANEXPANSION FACTOR OF FROM 1.5 TO 10, WHICH THERMOPLASTIC RESIN HAS ASOFTENING POINT BETWEEN 40*C. AND 180*C.